Ink-jet printers include one or more pens for delivering drops of ink to a printing medium, such as paper. An ink-jet pen typically includes a nozzle plate that has formed in it a plurality of nozzles. The nozzles are in fluid communication with an ink reservoir.
Any of a number of mechanisms may be employed for expelling ink through the nozzles of the pen. For instance, one mechanism, known as thermal-type ink-jet printing, includes a thin-film resistor mounted adjacent to each nozzle. To expel a drop of ink from a nozzle, a current pulse is applied to the resistor for heating the resistor. The heated resistor vaporizes a portion of the ink near the nozzle. The rapid expansion of the ink vapor forces a drop of ink through the nozzle. This "firing" of drops is controlled by a microprocessor in response to external data that is provided to the printer and that represents part of the desired image to be printed.
The ink-jet printer includes mechanisms for moving the pen and for advancing the paper relative to the pen. Typically, the pen is scanned across the paper one or more times, the paper is advanced, and the pen is again scanned across the paper. The microprocessor-controlled firing of selected nozzles at selected times during scanning of the pen produces on the paper an arrangement of ink dots in a resolution high enough to represent an image or textual information.
A measure of the quality of an ink-jet printed image is the uniformity of the printed ink density across the surface of the image. Preferably, individual ink drops will penetrate the permeable printing medium and diffuse evenly through the medium, joining with adjacently printed drops to form a continuous image element of substantially uniform ink density.
Unevenness in ink density causes an undesirable mottled appearance in the printed image. Uneven ink density may result in instances where adjacently printed ink drops randomly coalesce prior to penetration of the drops into the printing medium. This coalescence problem frequently occurs when the printing medium has low permeability, such as is characteristic of the clear films that are used for overhead projection displays.
Various methods have been used in the past to control ink drop placement for producing uniform, high-density images. For example, U.S. Pat. No. 4,748,453, entitled "Spot Deposition for Liquid Ink Printing," discloses a method wherein drops of ink printed in one scan of the pen are placed on the medium in a pattern that is intended to prevent overlap of flowable ink drops, thereby eliminating coalescence. A subsequent scan of the pen (which scan is delayed until the previously deposited drops sufficiently dry) prints new drops that overlap the previously printed drops. While this technique may be somewhat effective, the pattern in which the drops are printed during one scan results in diagonally adjacent drops being in tangential or perimeter contact. It has been found that this perimeter contact between simultaneously printed individual drops will cause coalescence of at least some of the diagonally adjacent drops. This uneven or random coalescence of ink drops will produce the mottled image mentioned earlier.
Placing drops on the printing medium in a manner that avoids any contact between simultaneously printed drops will eliminate the problem of drop coalescence; however, the overall ink density of the image will be reduced because of the corresponding increase in the amount of printing medium area that is exposed between drops, or more scans of the printhead over the printing medium will be required to achieve adequate ink density.